JP6638071B2 - Zirconium separation method and spent fuel treatment method - Google Patents

Description

  The present invention relates to a method for separating zirconium from a radioactive solution and a method for treating spent fuel.

  It is planned that spent fuel discharged from nuclear power plants will be processed into vitrified material after geological disposal after nuclear fuel material is recovered by reprocessing. Until now, the elements recovered in the reprocessing step have been uranium (U) and plutonium (Pu), which are nuclear fuel materials.

  However, in recent years, a minor actinide (MA), a radionuclide with a very long half-life, is recovered from spent fuel and transmuted into a nuclide with a short half-life, so that the radiotoxicity of the vitrified material is attenuated. The research to shorten the time significantly has been actively conducted. In addition, among fission products (FPs) contained in spent fuel, separation of fission products having a relatively long half-life (long-lived fission products: LLFP) has been studied. One of the LLFPs being considered for separation from spent fuel is zirconium (Zr).

  Patent Literature 1 describes an example of a method for separating a semi-volatile FP fluoride containing zirconium from spent fuel. In Patent Document 1, semi-volatile FP fluoride is aggregated on the surface of an aggregating material by a temperature difference in a flame furnace in which a nuclear fuel material and a fluorine gas are supplied and reacted to form a flame, and the residue is recovered. A method for sending to a department is disclosed.

In Non-Patent Document 1, in a nitric acid solution such as a high-level radioactive waste liquid, dissolved zirconium reacts with similarly dissolved molybdenum (Mo) to produce zirconium molybdate (ZrMo ₂ O ₇ (OH)). ₂ · 2H _{2 O)} is formed, that there is a property of precipitating as a solid have been reported.

  Non-Patent Document 2 describes data that the molybdenum concentration in the high-level radioactive liquid waste is lower than the zirconium concentration.

JP 2012-47546 A

Y. Kondo et al., Proceedings of Global 2009, 277-280 (2009) Review Separation and Transformation Engineering, Atomic Energy Society of Japan, page 4-2 (2004)

  In order to separate elements such as zirconium alone, it is necessary to separate elements from each other by utilizing the difference in chemical properties of the elements.

  Although it is possible to separate zirconium by the method described in Patent Document 1, it is desired that a specific element can be separated by a simpler method.

  Further, the high-level radioactive liquid waste generated by the wet reprocessing may be dried and solidified, and the solidified substance may be dissolved in a molten salt, and then the elements may be separated by an electrolytic method. This is called a molten salt electrolysis method.

  The present inventors have studied a method of recovering long-lived fission products (LLFP) element by element from high-level radioactive waste liquid. As described above, it is considered that the LLFP contained in the high-level radioactive waste liquid can be recovered for each element also by the method of drying and solidifying the high-level radioactive waste liquid and recovering the elements by the molten salt electrolysis method. However, if zirconium can be separated from high-level radioactive liquid waste by a simple method in advance, the work of separating zirconium when LLFP is separated by a molten salt electrolysis method or the like can be omitted, and a more desirable separation process can be constructed. Reached.

  An object of the present invention is to separate zirconium from long-lived fission products (LLFP) contained in high-level radioactive liquid waste by a simple method, and to facilitate recovery of other LLFPs.

  The method for separating zirconium of the present invention is a method for separating zirconium from a radioactive solution containing zirconium, and by adding molybdenum to the radioactive solution, produces a precipitate that is a reaction product of zirconium and molybdenum, Separate the precipitate from the liquid.

  ADVANTAGE OF THE INVENTION According to this invention, among long-lived fission products (LLFP) contained in a high-level radioactive liquid waste, zirconium can be isolate | separated by a simple method, and collection | recovery of other LLFP can be facilitated.

4 is a flowchart illustrating a method for separating zirconium according to the first embodiment. FIG. 1 is a schematic diagram showing a zirconium separation device of Example 1. 6 is a flowchart illustrating a method for separating zirconium according to a second embodiment. FIG. 5 is a schematic diagram showing a zirconium separation device of Example 2. 4 is a graph relating to the principle of the present invention and showing the result of calculating Gibbs free energy for the reaction between molybdenum oxide and fluorine gas and the reaction between zirconium oxide and fluorine gas. 4 is a graph relating to the principle of the present invention and showing the result of calculating the temperature dependence of the vapor pressure of molybdenum fluoride and zirconium fluoride. 4 is a graph relating to the principle of the present invention and showing the enthalpy of reaction when a fluorine gas reacts with an oxide of molybdenum, zirconium or uranium.

  The present invention relates to a method for separating zirconium from a radioactive solution and a method for treating spent fuel. Here, the radioactive solution is a high-level radioactive waste liquid or the like. The spent fuel is a spent nuclear fuel containing zirconium.

  As described in Non-Patent Document 1, zirconium precipitated as zirconium molybdate can be recovered from a high-level radioactive waste liquid (hereinafter, referred to as “high-level waste liquid”) as a solid precipitate. However, the data of Non-Patent Document 2 shows that even if molybdenum and zirconium in the high-level waste liquid react, not all zirconium precipitates. Therefore, a certain amount of zirconium continues to be dissolved in the high-level waste liquid.

  Therefore, the present inventor has focused on the reaction between molybdenum and zirconium, and has devised a method for separating zirconium from a high-level waste liquid by a simple method.

  Specifically, the first method is to additionally dissolve molybdenum in high-level waste liquid and react zirconium with molybdenum to precipitate all zirconium as zirconium molybdate, and to elevate the precipitate to a high level. This is a method of separating zirconium by recovering it from waste liquid.

  The second method is to concentrate the high-level waste liquid and increase the concentration of zirconium and molybdenum to make the reaction more likely to occur.

  However, with the second method alone, the amount of molybdenum is insufficient to precipitate all the zirconium. Therefore, combining the first method and the second method can be an effective method.

  Hereinafter, examples will be described.

  FIG. 1 is a flowchart of the present embodiment, and shows steps up to the separation of zirconium from spent fuel.

  In this figure, a method for separating zirconium includes a precipitation step S101 for adding molybdenum 6 to a high-level waste liquid 5 (a radioactive solution containing zirconium) to precipitate zirconium in the high-level waste liquid as zirconium molybdate. A precipitate collecting step S102 for collecting the generated precipitate 7 from the residual liquid 8, a drying step S103 for drying the collected precipitate 7, and a reaction between the dried precipitate 7 and the fluorine gas 11 to form a molybdenum fluoride 9 And a fluorination step S104 of obtaining zirconium fluoride 10. Further, the method for separating zirconium may include a conversion step S105 for changing the chemical form of molybdenum fluoride 9.

  Hereinafter, the method for separating zirconium of the present example will be described in more detail.

  In the reprocessing of spent fuel discharged from nuclear power plants, spent fuel is sheared, the sheared material is put into a nitric acid solution to dissolve fuel components, and uranium and plutonium are recovered from the solution by solvent extraction. . The waste liquid remaining after recovering uranium and plutonium is high level waste liquid. Molybdenum 6 is added to the high level waste liquid 5. The chemical form of molybdenum 6 is not particularly limited, but is preferably in the form of nitrate, oxide or metal in order to prevent unnecessary components from being brought into the solution. The recovery of uranium and plutonium may be performed after molybdenum is added to a solution in which spent fuel is dissolved to separate zirconium. In such a case, a solvent extraction method can be used.

  Further, considering the chemical form of zirconium molybdate, molybdenum 6 is adjusted and added so that the molar concentration of molybdenum in the high-level waste liquid 5 is at least twice the molar concentration of zirconium. Then, all the zirconium in the high level waste liquid 5 reacts with molybdenum to become insoluble zirconium molybdate and precipitates. This precipitate corresponds to the precipitate 7. The precipitate 7 is recovered from the solution. A general solid-liquid separation method can be applied to the recovery method. For example, a filtration method can be applied.

  According to the method described above, zirconium can be recovered as zirconium molybdate from a high-level waste liquid by a very simple method. The residual liquid 8, which is the high-level waste liquid 5 from which the precipitate 7 has been removed, is sent to a step for recovering LLFP other than zirconium remaining in the residual liquid 8, and is subjected to a treatment such as a molten salt electrolysis method. Is performed.

  Hereinafter, a method for separating zirconium alone from zirconium molybdate will be described.

  Since the precipitate 7 obtained in the above step is wet with the solution, it is dried in the drying step S103. For example, by heating to about 100 ° C. while blowing air into the container containing the precipitate 7, the water attached to the precipitate 7 can be blown off. The precipitate 7 thus dried is reacted with the fluorine gas 11 in the fluorination step S104. The fluorine gas 11 does not need to be pure fluorine, but may be a mixed gas of fluorine and an inert gas. As the inert gas, a rare gas such as argon, a nitrogen gas, or the like can be used.

  FIG. 5 shows the results of calculating Gibbs free energy for the reaction between molybdenum oxide and fluorine gas and the reaction between zirconium oxide and fluorine gas.

  From this figure, it can be seen that the value of Gibbs free energy of the reaction between these oxides and fluorine gas is a large negative value at any temperature, and it is very easy to react with fluorine gas. Therefore, it is considered that when a fluorine gas is brought into contact with these oxides, the oxides are immediately converted to fluorides.

  Molybdenum oxide and zirconium oxide differ in chemical form from zirconium molybdate. However, since oxides are very reactive with fluorides, zirconium molybdate, whose main constituent elements are oxygen, hydrogen, molybdenum and zirconium, also reacts readily with fluorine gas to convert to fluorides. Conceivable.

  The temperature at which the precipitate 7 reacts with the fluorine gas 11 is preferably set to 100 ° C. or more and 200 ° C. or less (range from 100 ° C. to 200 ° C.). When the temperature at the time of the reaction is 100 ° C. or higher, molybdenum fluoride 9 volatilizes and zirconium fluoride 10 remains solid, so that molybdenum and zirconium can be separated.

  The reason will be described with reference to FIG.

  FIG. 6 shows the calculation results of the temperature dependence of the vapor pressure of molybdenum fluoride and zirconium fluoride.

  It can be seen that molybdenum fluoride has a high vapor pressure, and if the temperature is 100 ° C. or higher, the vapor pressure exceeds 1 atm, so that it is easily scattered as a gas. On the other hand, the vapor pressure of zirconium fluoride does not exceed 1 atm unless it reaches about 900 ° C. or higher. Therefore, if heating is performed at a temperature of 100 ° C. or higher at a temperature lower than the heat-resistant temperature of a furnace material used in general, molybdenum and zirconium can be separated by utilizing a difference in vapor pressure. The heat-resistant temperature of a commonly used furnace material (for example, Ni alloy) is about 600 ° C. Therefore, by heating at 100 ° C. or more and 600 ° C. or less, molybdenum fluoride can be vaporized, and the remaining zirconium fluoride can be taken out as a solid. Here, the lower the heating temperature, the easier the operation is, and the separation operation is possible even at 200 ° C. or less, which is desirable.

  In FIG. 1, with respect to molybdenum fluoride 9, the chemical form is changed to something other than fluoride in the conversion step S105. The chemical form is preferably an oxide. Further, it may be a nitrate or a metal. The molybdenum whose chemical form has been converted may be used for the purpose of adding it to the high-level waste liquid 5 again.

  According to the method described above, zirconium can be independently recovered from high-level waste liquid.

  In the first embodiment, as an example, the target to which molybdenum is added is a high-level waste liquid. However, the target to which molybdenum is added need not be limited to high-level waste liquid, and molybdenum may be added to a solution in which spent fuel is dissolved in nitric acid, and zirconium may be separated as zirconium molybdate. Also in this method, zirconium can be separated as in the first embodiment.

  Further, before adding molybdenum to the high-level waste liquid, the high-level waste liquid may be concentrated in advance. This makes it possible to increase the zirconium concentration in the high-level waste liquid, thereby promoting the reaction with molybdenum and efficiently obtaining zirconium molybdate.

  Next, the zirconium separation device of the present embodiment will be described.

  FIG. 2 is a schematic diagram showing a zirconium separation device of the present embodiment.

  The zirconium separation apparatus shown in this figure includes a dryer 21 and a fluorination reactor 22.

  First, as described above, in the precipitation step S101, molybdenum 6 is added to the high-level waste liquid 5, and the precipitate 7 is recovered from the solution (the precipitation recovery step S102 in FIG. 1). The collected precipitate 7 is supplied to the dryer 21 (drying step S103 in FIG. 1). As the dryer 21, a rotary kiln type dryer can be used. The dryer 21 is provided with a heater 23, and maintains the internal temperature at about 100 ° C. by heating. The air 30 is supplied to the dryer 21 together with the precipitate 7. Thereby, the water content of the precipitate 7 evaporates. In this way, a recovered material 32 that is a dried precipitate 7 is obtained.

  On the other hand, the air 30 and the evaporated water are collected separately from the collected matter 32 as the exhaust gas 31. Thereafter, the recovered material 32 is supplied to the fluorination reactor 22 (the fluorination step S104 in FIG. 1). As the fluorination reactor 22, a rotary kiln type reactor can be used. The fluorination reactor 22 is provided with a heater 24, and maintains the internal temperature at 100 ° C or higher and 200 ° C or lower by heating. Then, the recovered product 32 and the fluorine mixed gas 33 are supplied to the fluorination reactor 22 and reacted therein. The fluoride mixed gas 33 is a mixed gas of a fluorine gas and an inert gas. As the inert gas, a gas that does not affect the reaction is used, for example, an argon gas or a nitrogen gas can be used.

  Due to the reaction inside the fluorination reactor 22, the recovered substance 32 is changed into molybdenum fluoride 9 and zirconium fluoride 10. At the temperature in the fluorination reactor 22, the molybdenum fluoride 9 is a gas, and the zirconium fluoride 10 is a solid. Therefore, the zirconium fluoride 10 can be recovered alone by separating and recovering the gas and the solid. In this way, zirconium can be recovered alone.

  According to this embodiment, zirconium can be precipitated as a solid by a very simple method such as adding molybdenum to a high-level waste liquid or concentrating the high-level waste liquid. Further, zirconium, which is a precipitate, can be recovered from the high-level waste liquid by a very simple method such as a solid-liquid separation method. Thus, zirconium can be separated from high-level effluents in a very simple manner as a whole. By combining this method with an existing LLFP separation method such as a molten salt electrolysis method, an effect of simplifying a separation process using the molten salt electrolysis method in LLFP separation can be obtained.

  FIG. 3 is a flowchart showing the steps of this embodiment until the zirconium is separated from the spent fuel.

  This embodiment has a configuration in which a separation step S205 is added to the zirconium separation process described in the first embodiment. Hereinafter, the method for separating zirconium of the present example will be described in detail.

  In the present embodiment, the method up to the fluorination step S204 is the same as in the first embodiment. The dried precipitate 7 obtained in the drying step S203 is reacted with the fluorine gas 11 in the fluorination step S204. Unlike Example 1, in the fluorination step S204, a flame furnace type reactor is used. Further, it is preferable to use a high-concentration fluorine gas of 90% or more. An example of a flame furnace type reactor is shown in Patent Document 1, for example. In a flame furnace type reactor, an object to be fluorinated reacts with high-concentration fluorine gas, so that a region where a reaction occurs due to reaction heat generated at the time of a chemical reaction has a considerably high temperature of 1000 ° C. or more. Thereby, the rate of the chemical reaction increases, so that the flame furnace type reactor has a high reaction efficiency.

  FIG. 7 shows the enthalpy of reaction when each oxide reacts with fluorine gas.

  The present inventors have confirmed by experiments that uranium oxide can be treated in a flame furnace. It can be seen from FIG. 7 that the oxide of molybdenum and zirconium, which is the object to be fluorinated in the present invention, also has a large negative enthalpy in the reaction with fluorine. Therefore, it is expected that zirconium molybdate (corresponding to precipitate 7), which is a compound of molybdenum and zirconium, can be similarly treated in a flame furnace.

  As described above, when the dried precipitate 7 obtained in the drying step S203 of FIG. 3 is reacted with the fluorine gas 11 in the fluorination step S204 of the flame furnace method, a region where the chemical reaction occurs due to the heat of the chemical reaction is formed. Since the temperature becomes extremely high at 1000 ° C. or more, both the generated molybdenum fluoride and zirconium fluoride become gas.

  The mixed gas obtained here is subjected to a separation step S205. The separation step S205 is a pipe-shaped container through which a gas can flow, and the temperature is kept at 200 ° C. or lower (the solid recovery section 41 in FIG. 4 (described later)).

  At 200 ° C. or lower, as shown in FIG. 6, only zirconium fluoride becomes a solid, so that in the separation step S205, zirconium fluoride precipitates as a solid, and molybdenum fluoride flows out as a gas.

  According to the method described above, zirconium can be independently recovered from high-level waste liquid. Further, in this embodiment, since both zirconium and molybdenum are temporarily used as fluoride gas in the fluorination step S204, unreacted solid hardly remains in the gas-solid reaction, and separation of molybdenum and zirconium is easy. Is obtained.

  Next, the apparatus for separating zirconium of this embodiment will be described with reference to FIG.

  The zirconium separation device shown in this figure has a configuration in which the fluorination reactor 22 is replaced with a frame furnace 40 and a solid recovery unit 41 in the zirconium separation device described in the first embodiment. The frame furnace 40 is provided with a heater 42, and the solid recovery section 41 is provided with a heater 45. The steps up to obtaining the recovered material 32 are the same as in the first embodiment.

  In this embodiment, the recovered material 32 is supplied to the flame furnace 40 together with the fluorine gas 43. At this time, the wall surface of the frame furnace 40 is kept at about 150 ° C. by the heater 42. Inside the flame furnace 40, a reaction between the recovered material 32 and the fluorine gas 43 forms a frame 44, which is a high-temperature reaction region, and a fluorination reaction occurs inside and near the frame 44. The gas generated by the fluorination reaction immediately exits the flame furnace 40 and flows into the solid recovery unit 41 in a form in which the gas is expelled by the sequentially supplied fluorine gas and the gas generated by the fluorination reaction.

  The solid recovery section 41 is kept at a temperature of about 150 ° C. by a heater 45. Therefore, the temperature of the gas that has flowed in is reduced, and only zirconium fluoride is condensed and separated from gaseous molybdenum fluoride. A Raschig ring or the like may be placed inside the solid recovery unit 41 to increase the area in contact with the gas so that zirconium fluoride can be easily condensed. The gaseous molybdenum fluoride is expelled by the inflowing gas and discharged from the solid recovery unit 41. In this way, zirconium can be recovered alone.

  According to the present embodiment, since both zirconium and molybdenum are temporarily used as fluoride gases in the fluorination step S204, unreacted solids are less likely to remain in the gas-solid reaction. And the effect of facilitating the separation can be obtained.

  S101, S201: Precipitation precipitation step, S102, S202: Precipitation recovery step, S103, S203: Drying step, S104, S204: Fluorination step, S105, S206: Conversion step, S205: Separation step, 5: High-level waste liquid, 6: molybdenum, 7: precipitate, 8: residual liquid, 9: molybdenum fluoride, 10: zirconium fluoride, 11: fluorine gas, 21: dryer, 22: fluorination reactor, 23: heater, 24: heater 31: exhaust gas, 32: recovered material, 33: fluorine mixed gas, 40: flame furnace, 41: solid recovery unit, 42: heater, 43: fluorine gas, 44: frame, 45: heater.

Claims (12)

A method for separating zirconium from a radioactive solution containing zirconium,
By adding molybdenum to the radioactive solution, a precipitate that is a reaction product of zirconium and molybdenum is generated,
Separating the precipitate and the liquid,
A method for separating zirconium, wherein the precipitate is dried, reacted with fluorine, and heated to 100 ° C. or higher and 600 ° C. or lower to separate molybdenum contained in the precipitate as a fluoride gas. A method for separating zirconium from a radioactive solution containing zirconium,
By adding molybdenum to the radioactive solution, a precipitate that is a reaction product of zirconium and molybdenum is generated,
Separating the precipitate and the liquid,
The precipitate, dried by Rukoto reacted with fluorine in a frame furnace, molybdenum and zirconium contained in the precipitate as the gaseous fluoride, by cooling the gas to 600 ° C. or less, the zirconium A method for separating zirconium, wherein the fluoride is made a solid and the molybdenum fluoride is separated as a gas. A method for separating zirconium according to claim 1 or 2,
A method for separating zirconium, wherein the radioactive solution is concentrated before adding molybdenum to the radioactive solution. A method for separating zirconium according to claim 1 or 2,
The method for separating zirconium, wherein the radioactive solution is a nitric acid solution. A method for separating zirconium according to claim 1, wherein
A method for separating zirconium, wherein a temperature at which the precipitate reacts with fluorine is 100 ° C. or more and 200 ° C. or less. A method for separating zirconium according to claim 2, wherein
The method for separating zirconium, wherein the gas is cooled to 200 ° C. or lower. A method for separating zirconium according to any one of claims 1 to 6,
The method of separating zirconium, wherein the molybdenum fluoride changes its chemical form and is added to the radioactive solution. A method for treating spent fuel, which is spent nuclear fuel containing zirconium,
By adding molybdenum to the solution in which the spent fuel is dissolved, a precipitate that is a reaction product of zirconium and molybdenum is generated,
Separating the precipitate and the liquid,
A method for treating spent fuel, comprising drying the precipitate, reacting it with fluorine, and separating the molybdenum fluoride as a gas at a temperature of 100 ° C. or more and 600 ° C. or less . A method for treating spent fuel according to claim 8,
A method for treating spent fuel, comprising removing uranium and plutonium contained in the solution before adding molybdenum to the solution. A method for treating spent fuel according to claim 8,
A method for treating spent fuel, comprising removing uranium and plutonium contained in the liquid after separating the precipitate and the liquid. A method for treating spent fuel according to any one of claims 8 to 10,
The method for treating spent fuel, wherein the solution is a nitric acid solution. A method for treating spent fuel according to claim 8,
Fluoride of the molybdenum, converted its chemical form, added before Ki溶 liquid processing method of spent fuel.

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